Although tapered plug valves may be cycled for operation by simply rotating plug members between the open and closed positions thereof, such rotation may require application of considerable torque to a valve stem. In many cases, valve actuator mechanisms have been employed that have the capability of imparting both linear and rotary movement to the valve element so that the valve may be unseated prior to rotation thereby allowing rotation to occur through application of minimal torque forces. Examples of valve actuator mechanisms incorporating linear and rotary components of movement are identified by U.S. Pat. No. Re 20,973 of Goldberg, et al; U.S. Pat. No. Re 22,455 of Wilson; U.S. Pat. No. 3,793,893 of Heinen and U.S. Pat. No. 3,908,697 of Witzel. In most cases, valve actuator mechanisms for imparting linear and rotary movement to a plug valve member are of fairly complex and this type require guiding means such as guide pins and grooves to insure that axial movement and rotational movement do not occur simultaneously so that accurate positioning of the valve element may be controlled. It is desirable to provide a valve actuating mechanism which is of simple and relatively inexpensive nature and yet which is capable of accomplishing accurate positioning of the valve element without any requirement for positive stem guide structure.
A problem that typically occurs when valve actuator mechanisms are employed that induce both liner and rotary components of movement to valve mechanisms, is the inability of such mechanisms to control such movement during all phases of valve actuation. In many cases, at the intermediate portions of rotary and linear movement, interacting mechanical parts become loose and a valve mechanism can be rotated by line pressure irrespective of the position of the valve actuator. This activity is typically known as "windmilling". In fact, a valve windmilling effect can be developed such that forces developed by the pressure of flowing fluid suddenly rotate the valve element through several degrees of free movement due to slack in the actuating mechanism. After this slack has been taken up by rotational movement, the valve actuator mechanism will then restrain the valve mechanism against further rotation. The valve element, however, slams from one position to another and this slamming force is introduced through the valve stem and into the valve actuating assembly. Under conditions where this windmilling induced slamming phenomena occurs, a valve actuating mechanism can become worn quite quickly to the extent that operational failure occurs. It is desirable, therefore, to provide a valve actuating mechanism which enables the provision of completely controlled movement during all phases of both linear and rotational increments of movement. Further, it is desirable to provide a valve mechanism that simultaneously introduces both linear and rotary components of movement to a valve stem for controlling actuation of a valve mechanism or other suitable mechanical device.